This invention relates to a tappet (valve lifter) installed between a valve stem and a valve actuating element in internal combustion engines. The hollow tappet houses, coaxially therewith, a closed hydraulic valve clearance adjusting system having a piston-like valve clearance adjusting element and including an elastomer diaphragm that extends from the inner circumference of the hollow tappet body to the outer circumference of the valve clearance adjusting element. The diaphragm is clamped at its beaded circular ends by clamping rings against the component to be sealed. In the zone of the seating face a circumferential groove is provided in the component to be sealed, for receiving an annular projection of the diaphragm.
A valve tappet of the above-outlined type is known and is disclosed, for example, in U.S. Pat. No. 4,397,271. In this prior art construction, the diaphragm situated in the annular space between the tappet and the valve clearance adjusting element is secured by means of clamping rings vulcanized into the bead-like end zones of the diaphragm. While the end zone of the diaphragm engaging the inner circumferential surface of the tappet is axially displaceable, the end zone of the diaphragm which engages the outer circumferential face of the valve clearance adjusting element is axially immobilized in a groove provided in the valve clearance adjusting element. It is a disadvantage of this type of securement of the diaphragm at the valve clearance adjusting element that the service life of the diaphragm is limited. On the one hand, the clamping ring vulcanized into the diaphragm bead is of such small dimensions that it is not capable of exerting high clamping forces and, on the other hand, the cooperating seating face of the valve clearance adjusting element is axially very narrow so that after a certain service period a failure of the seal in those locations must necessarily occur.
During operation, the valve tappet is exposed to high temperature differences and extremely high acceleration forces. The elastomer component which contracts or expands in response to temperature changes is not capable of sealing the associated seating face and thus, due to the high acceleration forces, a slippage of the diaphragm with respect to the seating face occurs. In order to prevent a relative displacement between the diaphragm and the valve clearance adjusting element, German Offenlegungsschrift (non-examined published application) No. 3,542,708 proposes to bond the valve clearance adjusting element with the diaphragm by vulcanization and to provide an annular projection in a corresponding groove of a bottom wall. In this construction, the diaphragm is coupled entirely rigidly with the valve clearance adjusting element so that in case of a temperature change or under the effect of fluid the changing volume cannot plastically deform. This causes stress peaks in the diaphragm which may cause rupture thereof.
It is a further disadvantage of prior art arrangements that after an extended service period the diaphragm vulcanized to the valve clearance adjusting element may become separated therefrom and a gap appears. The hydraulic medium then escapes through the gap to the base plate and subsequently flows past the projection of the diaphragm into the space between the base plate and the diaphragm. The axial pressure of the coil spring which is situated within the tappet and which engages the diaphragm is not capable of pressing the diaphragm about its entire circumference uniformly against the base plate since a coil spring is not adapted to exert a pressing force along the entire circumference.